Micropuncture studies were carried out in 19 dogs to examine the relationship between glucose and sodium transport under the effect of phlorhizin. In 13 hydropenic dogs, phlorhizin exerted a small effect on proximal sodium reabsorption but completely inhibited proximal glucose reabsorption. In 6 volume-expanded dogs, phlorhizin had no effect on the already inhibited proximal sodium reabsorption but again completely inhibited glucose reabsorption. It was concluded from these results that phlorhizin effect on glucose transport is independent of sodium transport changes and therefore at least one component of proximal glucose transport is specific to glucose. In the second series of experiments, the distal site of glucose transport was studied. Administration of acetazolamide in 9 dogs inhibited both glucose and sodium transport in proximal tubule but resulted in no glycosuria despite a significant natriuresis. In 7 dogs, pre-treated with acetazolamide and extracellular volume expansion, furosemide had no effect on the proximal glucose reabsorption but caused significant glycosuria in the final urine. It was concluded that glucose reabsorption at a distal site, including a possible site in the loop of Henle, plays an important role in regulating urinary glucose excretion and thus maintains renal threshold for glucose.